Cab signalling system for railroads



Jan. 17, 1956 c. F. STAFFORD CAB SIGNALLING SYSTEM FOR RAILRoAns 4 Sheets-Sheet 1 Filed June 30, 1950 m NDOI l DE@ nventor llll Jan. 5717, 1956 c. F. STAFFORD CAB SIGNALLING SYSTEM FOR RAILROADS Fiied June :5o. 195o 4 Sheets-Sheet 2 n SFD@ nnomo M0453 nventor n :ESO EEFHE .ONE

(Ittorneg Jan. 17, 1956 c. F. rSTAFFORD CAB SIGNALLING SYSTEM FOR RAILROADS 4 Sheets-Sheet 5 Filed June 30, 1950 Bnnentor lb Cttomeg DMDOU .0.4

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Jan. 17, 1956 c. F. STAFFORD CAB SIGNALLING SYSTEM FOR RAILROADS 4 Sheets-Sheet 4 Filed June 30, 1950 www mgl

L Gttorneg United States Patent() CAB SIGNALLING SYSTEM Fon RAlLRoADS Chester Friend Staiord, Rochester, N. Y., assigner to General Railway Signal Company, Rochester, N. Y.

Application June 30, 1950, Serial No. 171,424 y claims. (ci. 246-63) This invention relates to railway signalling systems and more particularly pertains to a cab signalling organization providing a display of signal aspects on a train to correspond to traic conditions.

In territories where cab signalling is to be used, current pulses occurring at distinctive rates are generally applied to the rails of the track sections. Each code rate corre sponds to a particular signal condition with the complete absence of pulses corresponding to the most restrictive indication. hese pulses of current cause receiving and decoding apparatus on the train to be intermittently energized by the inductive transfer of coded track current energy with the rate of such energization corresponding to the coding rate of the track current. apparatus then selectively responds to the rate of its energization and controls the cab signal causing it to display the proper signal aspect. v

On some railroads the current in the tracks is direct current, and on other railroads alternating current is used to energize the track circuits. On other railroads, the.'

track current may consist of rectied but unfiltered alternating current with the rectication being either full-wave or half-wave. Various other railroads may have different kinds of track circuit energization in diterent localities. Thus, a portion of a railroad may employ direct current or rectified alternating current and an adjoining territory may have alternating current track circuits. It is desirable, however, that the train-carried apparatus be operable over the entire railroad regardless of the kind of track circuit energization employed in the various portions of the railroad. Therefore, the code detecting and receiving apparatus of the present invention includes means for permitting proper operation with either alternating current, direct current, or rectilied alternating current track circuit energization. A two-position changeover switch is provided to cause the circuit organization to conform with the particular kind of track circuit energization encountered. With the switch in one position, the apparatus is conditioned to operate in regions of direct-current or rectified alternating-current track circuit energization; whereas, with the changeover switch in the other position, the apparatus is conditioned to operate in regions where the track circuits are energized with alternating current.

In addition to the adaptability of this apparatus to different kinds of track circuit energization, additional features are included to prevent the display of a signal indication less restrictive than stop under various conditions of failure of the apparatus. Thus, the use of two channels of amplification, for example, prevents improper operation in the event that various connections are inter-` mittently broken. Also, stray currents in the track rails are prevented from causing operation of the code detecting apparatus because of trap circuits included in the circuit organization.

Other objects, purposes, and characteristic features of the present invention will be in part obvious from the accompanying drawings and in part pointed out asthe description of the invention progresses. In describing The decodingv Patented Jan. 17, 195s ICC 1 the invention in detail reference will be made to the accompanying drawings in which:

Fig. 1 diagrammatically illustrates a portion of railroad track in which various kinds of track circuit energization have been shown in adjoining track sections; and

Figs. 2A, 3A and 4A each show an embodiment of the code receiving and decoding apparatus of this invention and the associated wave form diagrams of Figs. 2B, 2C, 3B, and 4B are provided to aid in the description of the manner of operation of the various embodiments.

The parts and circuits of this invention are shown diagrammatically and conventional illustrations are used to simplify Ithe drawings and explanations. The drawings have been made to make it easy to understand the principles and manner of operation rather than to show the specic construction and arrangement of parts that would be used in practice. Relays and their contacts are shown in a conventional way. The symbols (-1-) and indicate connections to the positive and negative terminals,

respectively, of a suitable source of direct current pref-l erably of a relatively low voltage.

The symbol B+ and the symbol for ground potential indicate connections to the positive and negative terminals, respectively, of an? other suitable source of direct current preferably of a rela# tively higher voltage suitable for the electrodes of the varv, ions electron tubes employed.v

played is at all times dependent upon the traic condi General operation a rate corresponding to thecoding rate of the track cur-- When operating in regions of direct-current or rec-- rent. tiiied alternating current track circuit energization, the changeover switch adapts the apparatus to this form of energization by providing a circuit organization substantially as shown in my co-pending application Ser. No. 171,423, tiled June 30, 1950; and no claim is intended to oe made in this application to any subject matter disclosed in such co-pending application. When operated in this manner, each track code pulse induces a voltage in the receiver coils which are so connected that the voltages induced therein add to each other. The receivers are inciuded in a parallel tuned circuit that is preferably resonant to a relatively low frequency as compared to the usual commercial power frequencies. Oscillations are thus set up by each application and each removal of a track code pulse as is explained indetail in the prior application cited. By properly damping these oscillations, a distinctive voltage output of one polarity is produced at the beginning of each track code pulse and another disrinctive voltage outputA of the opposite polarity is pro-V duced corresponding to each removal of a code pulse from the track rails. These distinctive voltages are then amplified and control the operation of the code-responsive relay in such a way that each beginning of a track code pulse actuates the relay armature to one position and each ending of a track code pulse similarly actuates the relay armature to the opposite position.

The code-responsive relay governs the operation of a decoding organization that includes electromagnetic relays selectively responsive according to the code rate of the track current. Various contacts of these relays are then included in circuits that selectively energize the lamps of the cab signal. Thus, the signal aspect displayed in the cab is dependent upon the coding rate.

Also, since the coding rate of the track current in any track section is governed by the associated wayside apparatus to correspond to particular traiiic conditions in advance of such track section, the cab signal aspect distions in existence.

antw

Y track circuit energization .is used,.thc.changeover ,switch is moved to the corresponding position. This action then V adapts the circuit so that the Areceivers are included in a parallel'tunedcircui-t resonant Vtothe irequency'of Athetrack current source.Y Any suitabietra'ckicurrent frequency may beernpieyed although arfrequency of l-OO cyclesper-second is common and 'Will be lassumed to be the frequencyof kthe track currentv in 'the Idescription Athat follows.

VTlifhe alternating current inthe 'tunedcircuit is rectified,

j preferably byfaffull-Wave, .bridge-type rectiiierrto form a drectacurrentpulse corresponding to each track current pulse. 'I'h'esev resulting .direct-curreritpnlsesare `then Vacted upon by additional -plse-ifornaing'circuit V"means Isothat distinctive :voltage variationsof opposite polarity occur respectivelyatheb'eginningfand: 'end ofeac'h 'track code pulse. Jlhes'eldistinctive voltage-variations are amplified posite positions -fa't alrate corresponding to the rate of track Ycurrent coding. Y

ln one embodiment of thiszinvention, the pulseforrning means usedzto provide thedistinctiveoutput'voltages in re- Yspouse tothe coded alternating track 'current includes -another parallel tuned circuit. Thistuued vcircuit =is shockertcited'at the beginning and end olfeach track code pulse by the vpulseslforn'nedoy the rectification-of the alternatingcurrent pulses induced inthe receivers. `Each shock'excitation `initiates an oscillationinthe tuned circuit. 'By properly-darnping this ztunedcircuit,-Clistinctive output voltages are formed at the end and beginnin'gfofeachtrackfcode' pulse with successive variations of opposite polarity. Y'

in the two other embodiments shown, the pulse forming is accomplished by the charging ofcondensers through associated resistors. v.ln :one form ofthe invention :the

Yrectier output current is'applied directlyto the pulse forming means. In rthe other embodiment the rectifier outputpulses are rst.ampliedbeforebeing .applied'to the pulse-forming circuit means.

Track circuit energizaton decoding apparatus to variouskinds 'of ltrackecircuit energ'ization,V thek track sections llTfand 2T-have lbeen shown energized by unidirectional current; ispecically, VAdirect currentisshowmapplied to theV track rails.

fin track section 1T, a`tra'ck battery 15,variable-'currentlimiting-resistor 16 and coding contact 17 vare connected across the ftra'ckfrails. Asjcoding contact V17 is`-intern1it tehtly actuated by the Wayside'fappar'atus '12, :eachfclosurc of this :frontlcontact causes adirectecurrentfpulse foibe applied toltlierails. he'orm'of"the'resulting'track cur-A rent is "di-agranirnatically indicated above these Atrackeections. in Eig. l. Similar'apparatusis provided V'to apply coded direct current torthelrails 'of-track'sectionZT. -The delay in the riseY and decay of track current vat the vbeginnings'and ends offthe track pulses respectively lis caused by the track circuit characteristics including the rail resistance and inductan'ce and Ithe shunt vcapacitance yand conductancearidvaries therefore -vviththe distance from theieedV end as well as with changes"inlballastconditions. Y

v f'Ihe-d'ott'ed line drawnbetweenthewaysideapparatus and thertrackfse'ction ahead (denotes thatthefoperation'loi' this ywayside V4'apparatus lis"depender-1t jupon 'traliiciinadvanceoftrack =sectionffT A `Similar'"dotted"lines 'connect the'waysideappaatusdfblocks 'T8 'and 20 with `the-'track sections infadvance. l i y l The circuit providing for the'energization of'track secti'onli` includes an alternator 10 conne'ctedin series with the primary Winding' Vof a transformer 'Tfandv 'a coding 4 Y Y selective rate corresponding to traic conditions and also includesapparams .governing the operationof the .associated wayside signal 4. The secondary winding of trans former T is connected through variable current-limiting resistor 14 to the rails of track section 3T.

The manually operated selector lever associated with the Cab receiving and decoding apparatus oftrain TNI Y must, during the time that this train occupies track sections lTand 2T, be in theV position that conditions .the traincarried apparatus for operation with direct track current. Then, as indicatedfin Fig. l, ythe selector lever must be operated lto its vopposite vposition as trainTNly enters track section 3T where .alternating track current is employed,

As explained, the circuit organization Aprovided for operation With direct track current operates in a manner fully described in my co-pending application Ser. No. 171,423, led of even date herewith. This co-pending application points out that proper operation of the vehicle-carried apparatus fis "also 'obtained when 'full-wave or'ihalfwavefrectilied-alternating -track current is-used. Thus, although direct-current` track energization 'is shownin Fig. fl -for track circuits 1T and 2T, rectified*alternatingfcurrent could instead be applied lwithoutjrcquiring that the selector lever bef-foperatedroni the position -it ris fin Awhen direct track current :is used. y Y Y @parution-Fig; 2A Y in thefcmbodimentof the invention'shownrin Eig. 2A,

l the selector leverfSL andassociated :relay .'SRadiust the circuitfor;operation infeither regions of alternating Atrack currentorin .thoseregions in ywhich unidirectional track currentis employed. `When the selector :lever SL lis in theA pn'sitionshown -insEigL ..2A, vselector .relay/SR is deenergizedland'iits backzcontacts', 126, 27, zand 28 are closed. The circuit organization thus `,provided connects condenserS vin parallelwith receivers-3i) and 31 which aremounted in .inductive relationship to the track` rails. 'llhle magnitudekof :condenser 29 Vis preferably of such a valueia'sitotnnethe receivers 3i) andi to a relatively .low

frequency as;cornpared to Ythe `usual power frequencies. lninnerembodiment, .satisfactory operation was obtained byituning the :receivers .to approximately i4 cycles lper second.

*.Cohdenser-.ZQSis shunted by series-connected equal resistors 32and133 whosejunctionisconnected to ground.

I Thegremaining terminals lof resistors 32and 33 are each connectedthrough a backrcontact 'of relay SR and a corre- Sptondin'ggrid resistor rtofthc control grid of `anelectron tube.'` Ihefupperterrriinai of resistor 32, :for example, `is connected nhroughbackcontact? of relay `SR and 'resistor 34ato'sthe -control, gridotuhe'f. The ylower'terminal of ycillation.,ofthe ituned circuit With.the rs't half-cycle of thez-oscillationl-occurring.atthe beginning of a code pulse beingof opposite-fpolarity-to :the rst half-cycle of the oscillationioccurring-at theend of a code tpulse. 'When thesew oscillationstare-l critically ldamped;only a Asingle rolt-Y age`-var'iationlis 1retained ffrorneeach wave train and Lthis' single-"voltagei-variationicorrspdnds tothe Lfirst half-cycle ofsuchrwavetrain thus-providingan output voltage'across- Fig Inbpractice-'ithasibeenrfound thatcthe damping *Ina be ornewhat Elessthan-critical AThe advantage contact 19 whoseoper-ation Aisggoverne'd V'bythe `Wayside Y apparatusv 20. .This wayside Yapparatus may be .of Vany gaindbyfl'ess than, critical damping fisrau increased amplitildeof'ithe rdesired'voltagefvariation. A, slight voltage variation correspondingto. the second half-cycle o f .the train" 'of jwavesis'; retained .when the dampingisjless'than critical.' 'Satisfactory operation is still obtainedhowever,

brieruplayinsrsuceut damping to 'keep .the amplitude' of these secondary voltage variations substantially below a level causing relay operation, thereby providing an output voltage across the receivers such as that shown in waveform C of Fig. 2B. As already explained, suitably distinctive output voltages would also be obtained with this circuit organization if rectilied alternating track current pulses were applied to the track rails. The detailed explanation concerning the operation of this circuit arrangement provided for use with coded unidirectional track current is included in my co-pending application Ser. No. 171,423, filed of even date herewith.

Tubes 35 and 37 are each preferably provided with class A cathode bias by resistor 38 connected from the cathodes of these tubes to ground. Cathode resistor 38 is by-passed by condenser 39 which is of sucient size to minimize degenerative effects of the input signal. Since the junction of resistors 32 and 33 is connected directly to ground, and the cathodes of tubes 35 and 37 are also connected through the bias resistor 38 to ground, the voltages appearing on the grids of these tubes are of opposite polarity and each voltage is substantially equal to one-half of the voltage appearing across the parallel tuned circuit because only small voltage drops appear across the grid resistors 34 and 36 and the cathode resistor 38. Thus, the result is that at the beginning of each track code pulse the control grid of either tube 35 or tube 37 is driven less negative at the same time that the control grid of the other tube is driven more negative as illustrated by waveforms D and E of Fig. 2B. At the end of each track code pulse, the control grid of the tube previously made less negative is now driven more negative; whereas, the control grid of the tube previously driven more negative is now driven less negative.

A detailed description of the manner of operation of the balanced amplifiers including tubes 35, 37, 69, and 72 is presented in my co-pending application Ser. No. 171,423, tiled of even date herewith, wherein a similar circuit organization is provided for use with unidirectional current track pulses. The full description concerning the manner of operation will, therefore, not be repeated here. Briey, however, the manner of operation may be summarized as follows: When the grid-cathode voltage of either tube 35 or 37 is made less negative and the grid-cathode voltage of the remaining tube simultaneously more negative at the beginning of a track code pulse, the plate voltage of these tubes decreases and increases respectively. The grid-cathode voltages of the corresponding output amplifier tubes then become more negative and less negative respectively. If, for example, the grid-cathode voltage of tube 35 is made momentarily more negative at the beginning of a code pulse, the plate voltage of this tube will be momentarily increased, thereby also momentarily increasing the grid-cathode voltage of tube 69. At the same time the grid-cathode voltage of tube 37 will be made less negative, resulting in a plate voltage decrease and a corresponding decrease in the grid-cathode voltage of tube 72. Tubes 69 and 72 are both normally cut off because of the xed bias voltage supplied by battery 75. The fact that the grid-cathode voltage of tube 72 becomes more negative as described cannot aiect the conduction of this tube. However, the rise of grid-cathode voltage of tube 69 above cutoff does cause plate current to liov/ through the upper winding of relay CR, actuating its armature to a corresponding position. At the end of a track code pulse,` the conditions are reversed. The gridcathode voltage of tube 35 is made less negative, thereby momentarily decreasing its plate voltage so as to make the grid-cathode voltage of tube 69 more negative. The grid-cathode voltage of tube 37 is made more negative, however, thereby increasing its plate voltage and the gridcathode voltage of tube 72. This time, therefore, tube 69 remains cut olf, but tube 72 now conducts plate cur- 6 actuated to opposite positions upon successive beginnings and ends of the code pulses in the track rails. The decoding apparatus, shown in Fig. 4A, distinctively responds to the rate of actuation of relay CR and causes the proper cab signal aspect to be displayed.

As shown in Fig. 2A, a series trap circuit is between the plate of each of tubes 35 and 37 and the source of plate potential. Although these trap circuits are primarily included for use when the apparatus is used in areas of rectified alternating track currents, they may be highly useful in preventing interfering alternating currents of any frequency from adversely aiecting the operation of the train-'carried apparatus. A detailed explanation concerning one Vapplication of these trap circuits is presented in my co-pending application Ser. No. 171,423, tiled of even date herewith.

Resistors 34- and 36 are included to prevent improper operation of the circuit under certain conditions of malfunctioning of the apparatus. If, for example, an intermittent short circuit were to occur between the control grid and plate electrodes of tube 35, the direct voltage then applied to the parallel timed circuit would initiate transient oscillations. The removal of the direct Voltage would also initiate these oscillations. These oscillations might then be amplitied and cause false operation of the code-responsive relay CR. This possibility of erroneous operation is prevented by the inclusion of grid resistors 34 and 36. These resistors present a relatively high resistance to the direct voltage as compared to the resistance presented by the parallel tuned circuit. Thus, during a short circuit between control grid and plate of tube 35, most of this direct voltage appears across resistor 34, and the amount of this voltage appearing across the parallel tuned circuit is then of insucient amplitude to produce transient oscillations of great enough magnitude to cause operation of relay CR.

When the selector lever SL is operated to its dotted line position, a circuit is completed to energize the selector relay SR. With front contacts 25 and 26 of this relay closed, a condenser 45 and a rectifier 46 preferably of the kind providing full-wave rectification are connected in series across receivers 3l) and 31. This condenser is chosen to be of the proper size to tune the receivers to the track current frequency which is assumed in this description to be cycles per second.

The circulating current in the tuned circuit including receivers Sil and 31 and condenser 45 caused by the coded track current illustrated in waveform A of Fig. 2C is rectilied by full wave rectifier 46. The resulting rectiler output includes sine loops on successive half-cycles as shown in waveform B. This output of rectifier 46 is applied tothe primary winding of a transformer T1. The secondary winding of transformer T1 has connected across it a condenser 47, thereby providing a tuned circuit having a natural frequency relatively low as compared to the usual power frequencies. For purposes of analysis, to obtain a possible theory of operation concerning this embodiment of the invention, this output of the rectifier may be consdered as comprising a direct-current component plus a number of alternating-current components. Although the response of the tuned circuit comprising the secondary winding of transformer T1 and condenser 47 is actually caused bythe composite current in the tracks as shown in waveform B, the response of the circuit organization may be considered as being the sum of the various respouses produced by the corresponding components included in the rectiier output.

The predominant components in the full wave rectier output are a direct-current component and, if the track current is at 100 cycles per second, an alternating-current component of 200 cycles per second. Various other components at 400 and 800 cycles and still other higher frequency harmonics also appear but the amplitudes of y p these higher harmonics become progressively smaller so l that their elect is correspondingly small. i

T7 Dining the time that th-direct-current component vis steadily applied ato the transformer primary Winding, .no

eiect is produced inthe secondary windngbecause there tuned circuit are Vso chosen ,as to make this circuit oscillatory, transient Voscillations are initiated-in the tuned circuit each time that a voltageis induced in the secondary winding-as a result ofthe direct-.current component.k These oscillations Voccur-.at a rate corresponding to the' natural v.pariodof osciilation .of .the .tuned circuit.

"The predominant alternating-current component present during ,the timeof each code pulse in the tracks produces .a continually var yi genux ,in .the vpimary Winding, thereby inducing a corresponding voltage in Vthe secondary `winding. The rapid change lof the direct-current component at the vl'leginning and end of each code pulse produces the transient oscillations already described. Therefore, 'the composite receiver Aoutput voltage occurring during 'the on' period of the code when the trap circuit including condense-1' t9 and inductancey '50 and also resistors 65and 66ers Aremoved includes Vthe relatively Vhigh frequency alternating-current variations with Vthe average valuethereofvaried according to the'transient produced byV the direct-current component as is shown in Waveform Cof Fig. 2C. VSince the predominant rectifier ripple component Vis of a frequency twice that of the VtrackV current frequency, the main alternating component of the receiver output is 4also of'this frequency and its average value varies in a sinusoidalrapp'earing manner withthe ythe direct-current component again causes oscillations in the tuned circuit. These oscillations, however, are free of the higher Afrequency ripple because the alternatingcuirent component that produces the ripple'during the on period of the code 'is not present during the code ot period. The oscillations during the ott period are'also attentuated by the resistance inherently present inthe tuned circuit. The first half-cycle of oscillation of the average value occurring at the end of each track code pulse and subsequent half cycles'aswell are of the opposite polarity .as compared with corresponding half cycles of thc oscillations initiated at'thc beginningof each track code-pulse las `is shown Vin Waveform C.

A waveform of-voltagc across the Vsecondary tuned circuit having substantiallythe form shown in waveform C of Fig. 2C does not provide a completely'suitable input for the amplifying organization controlling the coderesponsive relay CR. Thewaveform of this voltage is'con- Sider-ably improved, however, by'adding damping *resisters 65 and 66 shown in Fig. 2A. lf resistorsY e5 and Cal 66 are chosen tobesofl such a value'that critical damping l of thel oscillations-is obtained, onlya `singleivariatio'nof, the averagevaiueis retained at `the 'beginning'of each code pulse and'another' slnglevariation ofoppo'site:polarityatj'the Yendof cachcotle pulse. `The distinctive voltage variations 'thus 'obtained to indicate "the beginning and end of each track code pulse correspondto the'rst halfcyclcs of the trains of oscillations set up respectively at the beginning and'ends of the ltrack pulses and may, under certain conditions, be energizing the balanced arnplier organization to control relay CR. Experiments showfthat'the damping provided by resistors 65 and on may .be n less Ythan'that amount of .damping Vknown as critical damping'thereby retaining to a small degree the second half-cycle of?V .eaehirain fof vIuscillaticns-; end arf viding an output -voltagezsirnilar to-thatshown lin wavefornrD.v If `the dampingis just .le-ss than critical,l this second half ycycle will not beV of sufficient amplitude to. obtainralse -respons'eof relay v( )Rrbut lthe advantageY gained isa greater amplitude of the-rst half-cycle thus insuring proper operation of .the apparatus with llower values of4 track current. Thus,`in .one embodiment, 'resistors and 66fwerechosen to be of twice-the valueA required forv vcritical damping thereby obtaining satisfactorily distinctive voltage outputs toV denote the .beginning and end of each codepulse.

The ripplefpresent the voltage across the tuned ytircuitwhen the trap circuit is .omitted is of suiiicient arnpli'tude 4asshowu in waveform VD that it will,` upon arnplication, causen chattering of the relay CR. "fo-prevent this condition, resistor 4S, condenser 49, and ir.-V

ductance Sil are includedas a trap circuit. Condenser 49 and inductance 50 arechosen tobe series-resonant at,A

the frequency of the ripple voltage. lzrorreaarnple, the frequency ofthe tracltctnrcnt is 100 cycles persecond, the most important .component of thisv ripple will have a frequency of 200 cycles per second because of the full-wave rectification employed and the series-resonant circuit-Will then be tuned` to this latter frequency.y Consequently, this resonant circuit presents a very low impedance to lthe 200 cycle ripple voltage, but presentsa very Yhigh impedance to other frequencies. At the 260 cycle frequency, the impedance of theV resonant circuit?. is'small as compared to the magnitude of the resistor 43 and, therefore,only a negligible amount of the'ripple voltage is applied through front contacts T7 and 2i? and cuit and damping resistors ancl e6 not only providesV suitably distinctive output'v'oltages to indicate the beginning'and end of each code pulse but also contains but asmall 'percentage of Fig. 2C. To summarize, the operation of this circuit organization lis such that a voltage variation of one polarity appears across series-connected resistors 65 and 66 at the beginning of Veach track code pulse, and another' voltage-variation of opposite polarityy occurs at the end of-each track lcode pulse. Y

Since the junctionofdamping resistors o5 and "ee is connected through commoncathode resistor to the cathodes of both` tubes-35 and 37,-the grid-cathode-'voltages oftubesBS and 37 are offopposite polarity. Also, grid resistors-'54 and 36 and cathode resistor 33 shunted by condenser 39 present a relativeiyflo.' impedance-to the input vsignal. as compared to the impedance presentedv sistorsfandl-respectivelymhecauseresistors i55 and e, donnent equal magnitndeetl-ie grid-cathode"taalt-ages-'ofY tubes 3S andsl -althoughJ-ot'oppositefleolarit have substantially uequal amplitrides.

'Resistorsfi V:and -E56-"areinclud-ed lin series with the control .grids of tubes 35 'and 37 respectively regardless offwhether relay SR-.is energized or deenergdzcd.- when the apparatus is adapted V'for operation in response to` alternating-current; track pulses, these resistors till the sameY requirement Vas that explained in connection `with operation of the apparatus `with direct-current or rectiiied alternatingfcurrent trackqblllscs; in brief, they prevent of ripple asshovm lin waveform E against improper responses of the input circuit in the event of intermittent short circuits between plate and grid of either tube 35 or 37.

At the beginning of each track code pulse, the gridcathode voltage of either tube 35 or 37 is made less negative at the same time that the grid-cathode voltage of the other tube is made more negative, Since, as already mentioned, tubes 35 and 37 are preferably provided with class A self bias, these tubes may conduct more and less plate current as their grid-cathode voltages respectively are made less negative and more negative. Assume, for example, that tube 35 conducts more current and tube 37 less at the beginning of a track code pulse. The momentary increase of tube 35 plate current through load resistor 67 then causes a corresponding momentary decrease of the plate voltage of this tube. The negativegoing voltage variation thus produced on the plate of tube 35 is applied through coupling condenser 68 to the control grid of tube 69. At the same time, tube 37 conducts less plate current thereby producing a corresponding increase of its plate potential because of decreased current through load resistor 70. Consequently, a positive-going voltage variation is then applied through coupling condenser 71 to the control grid of tube 72.

The control grids of tubes 69 and 72 are connected through the associated grid-leak resistors 73 and 74. respectively, to the negative terminal of a source of bias potential denoted by battery 75. Tubes 69 and 72 are preferably of the sharp cutott' variety and the bias applied to their control grids byl battery 75 is suicient normally to cut ott the plate current of these tubes. The cathodes of tubes 69 and 72 are both connected to ground. The plates of tubes 69 and 72 are each connected through a corresponding winding of polar relay CR to a source of positive potential designated B|-.

Under the assumed set of connections, the grid-cathode potential of tube 69 is decreased at the beginning of a track code pulse, and the grid-cathode potential of tube 72 is simultaneously increased. The decrease of tube 69 grid potential can have no effect upon the conduction of this tube since it is already cut oi. The increase of grid voltage of tube 72, however, now permits this tube to conduct and the resulting plate current through the lower winding of relay CR then actuates this relay to a corresponding position.

Under the same assumed conditions, the end of a track code pulse causes tube 35 to conduct less plate current and tube 37 to conduct more plate current. Therefore, the plate voltage of tube 35 will rise at the same time that the plate voltage of tube 37 decreases. A corresponding increase of voltage then appears on the grid of tube 69 and a decrease of grid-cathode voltage of tube 72. This time the conduction of tube 72 is unaffected, but with the grid-cathode voltage of tube 69 raised above cutoff, this tube now conducts and the resulting plate current through the upper Winding of relay CR now actuates this relay to the opposite position. As a result, relay CR is actuated iirst to one position and then to the other at a rate corresponding to the rate at which code pulses are applied to the track rails, As indicated in Fig. 2A, the decoding apparatus associated with the contacts of relay CR may be of the form shown in Fig. 4A and described in connection with that drawing.

Operation-Fig. 3A

In the embodiment of the cab receiving and decoding apparatus shown in Fig. 3A, the position of selector lever SL determines whether the circuit organization is adapted to operation with alternating-current track circuits or with unidirectional current track circuits. When the selector lever SL is in the position shown in Fig. 3A, relay SR is deenergized and its back contacts 76, 77, 78, and 79 closed. The circuit organization thus provided connects condenser 80 across the series-connected receivers 81 and 82 mounted adjacent the track rails, thereby tuning these receivers to a relatively low frequency with respect to the usual commercial power frequencies. The remainder of the circuit organization thus provided by the dropping away of relay SR for operation with directcurrent track pulses is the same as is shown in Fig. 2A. A trap circuit including condenser 96 and inductance 97 is connected between the plate of tube 162 and B+. A similar trap circuit including condenser 98 and inductance 99 is connected between the plate of tube 103 and B+. These trap circuits fulfill the same requirement as do the corresponding trap circuits explained in connection with Fig. 2A. The manner of operation of this form of the invention in response to direct track current is the same as that of the similar circuit organization of Fig. 2A as it operates in response to direct-current track pulses in that distinctive voltage variations are provided at the beginning and end of each track code pulse with successive voltage variations of opposite polarity. These voltage variations are then amplied as was explainedin connection with Fig. 2A and cause the code-responsive relay CR to be actuated alternately to opposite positions at a rate corresponding to the coding rate of the track current.

When the selector lever SL is operated to the dotted line position, relay SR is energized and closes its front contacts 7 6, 7'?, 7S, and 79. Condenser S5 and full-wave rectifier 86 are then connected in series across the seriesconnected receivers 81 and 82. Condenser 85 is preferably of the size required to tune the receivers to a frequency equal to the frequency of the alternating track current which has been assumed to be of 100 cycles per second and is diagrammatically illustrated by waveform A in Fig. 3B. The 200 cycle ripple component of the rectifier output is filtered by condenser 87, and resistors 88 and 89 connected across the rectifier output terminals, thereby providing a voltage across condenser 87 having ripple superimposed upon the direct-current component as shown in Waveform B.

In this form of the invention, no loose inductive coupling is provided as in the embodiment shown in Fig. 2A to reduce the load on rectiiier 86. A steady load appears across the rectiiier output terminals by reason of the current drawn by resistors 8S and 89. This relatively heavy load on the rectiiier output has the effect of reducing the selectivity of the tuned circuit including the receivers 81 and 82 and condenser 85. To prevent stray interfering track currents of frequencies other than that of the coded track current from being amplified by the cab apparatus, condenser 90 and inductance 91 are connected in series across the input terminals of rectifier 86. Condenser 90 and inductance 91 are preferably tuned to series resonance at a frequency equal to that of the predominating interference. Thus, in regions where power lines are located along the right-of-way distributing 60 cycle power, this trap circuit including condenser 90 and inductance 91 would preferably be tuned to this 60 cycle frequency to 'present a low impedance path for currents of this frequency thereby preventing their appearance, to a substantial degree, in the rectilier output.

To provide distinctive voltage variations at the beginning and end of each track code pulse, pulse-forming circuit means including resistors 88, 89, 92 and 93, and condensers 94 and 95 are provided. This pulse-forming circuit may be said to operate on the principle of a differentiating circuit as will be explained.

The directions of the arrows associated with the fullwave rectifier S5 indicates that the voltage at point 14H) is made positive with respect to voltage of point 101 as a result of each track code pulse. During the o period of the track code prior to any on period, any charges previously placed across condensers 94 and 95 reduce to zero because of the discharge path of these condensers provided by resistors 88, 89, 92, and 93. As the voltage of point 109 is suddenly made positive with respect to that at point 161 at the beginning of a track code pulse, charging current for condensers 94 and 95 passes through resistors 92 and `93 in "suche a direction as to tn ake theV Vvoltage at the junction of condenser 94 and resistor 92 p cathode voltage of tube 102 is suddenly made less negative as compared to its normal cathode bias voltagerat the same time that the grid-cathode voltage of tube 103 is made more negative. The charging current for condensers 94 and 95 is at lirst maximum but decreases as the voltage across each of these condensers increases. Therefore, the relatively sudden increase in average value of the voltage across resistors -92 and '93 at the beginning of each'track pulse is followed by a decrease of voltage with the rate of the decrease being dependent upon the time constant of condensers 94 and 95 associated with 'resistors 92 and 93. A short time constant produces a relatively narrow voltage variation; whereas a long time con-V stent brings about a corresponding increase' in the dura* y tion of the voltage variation.

At the end of each track code pulse another voltage variation of opposite polarity is produced. These voltage variations occurring as a result of the removal of track current are free from thedistinctive ripple caused by the alternating current because the alternating track current is, of course, not present in the rails at that time. In summary then, the voltage across resistors 92 and 93 as shown in waveform C includes a distinctive voltage variation atthe beginning of each track codeV pulse and another distinctive voltage variation of opposite polarity at the end of'every code pulse.

Tubes 102 and 103 of Fig. 3A are provided with class A bias by means of cathode resistor 104 which is bypassed 'oy condenser 10S. Grid resistors 106 and 107 arerhere also provided to prevent spurious responses of the input circuit as explained in connection with Fig. 2A in the event that intermittent short circuit should occur between grid and plate of either tube 102 and 103. The remainder of the circuit shown in Fig. 3A operates in the same manner as'thatv shown in Fig. 2A. Connecting the cathodes of both tubes 102 and 103 through resistor 104 and condenser 105' to the junction of equal resistors 92 and 93 causes the grid-cathode voltages of these tubes to be equal and of opposite polarity. Thus, as the grid cathode voltage of tube 102 is momentarily made less negative, the grid-cathode voltage of the'associated tube 103 is momentarily driven more negative. The subsequently voccurring input to these tubes reverses the situation with the grid of tube 102 being driven momentarily more negative at the Sametime that the grid of tube 103 becomes less negative. Each time that the grid voltage of either tube 102 or 103 is made less ueagtive, its plate voltage decreases because of the Vincrease of plate current through resistor 115. As the grid voltage of either tube becomes more negative,rits plate voltage increases because of its decrease of plate/current.

Tubes 116 and 117 are both biased to Vcutoff by reason of their grids being connected through resistors 118 and 119, respectively, to the negative terminal of a source of bias voltage 120. Therefore, a decrease of voltage, for example, applied from the plate of tube 102 through coupling condenser 103 tothe grid of tube 116 cannot alect the conduction of this tube. However, the simultaneously occurring increase of tube 103 plate voltage through coupling condenser 109 to the grid of tube 117 increases the grid voltage of this 'tube 117 so that it conducts plate current through the lower winding of relay CR. The armature of relay CR is then actuated toa corresponding position. The next input applied 'to tubes 102 and 103 causes a decrease Vin tube 103 plate voltage and an increase in tube 102 plate voltage which then causes a corresponding'increa'se of tube 116 grid voltage. Tube 116 then conducts plate 'current'through the upper winding of relay CR andthe armatureof this Arelay is then actuated to the opposite position. In thistway, relay CR is alternately actuated to one position 'and' then -to the other at a rate corresponding to the coding rate ofthe track current. This embodiment shown in Fig. 3A ordinarily hasY associated therewith decoding apparatusv which may be of the form shown in Fig. V4Aand has, therefore, only been indicated diagrammatically in Fie. 3A. The function of such decoding apparatus is, of course, to control theV cab 'signal so that it will display the proper signal aspect according to the rate of actuation of relay CR and in turn, therefore, according to the existing traic conditions:

Operation-F ig. 4AV

A third embodimentof the cab vreceiving and decoding apparatus is shown in Fig.- 4A. The manner of operation of this circuit organization is in general somewhat similar to that shown in Fig. 3A. A .major point of dierence,

however, is that the output of therectitier is applied after filtering directly to the' control grids of the firstY stage of amplification. The pulse-forming means isl included between the plates of the tubes included in this first stage and the control grids of the tubes includedV in the second stage of amplification.

With the selector lever SL in the' position shown so that relay SR is deenergized, back contacts 125, and 126Y of this relay are closed and front contact 127 is open. The series-connected receivers 128 and 129 thus have connected across them condenser 130 which provides a paralwhich is ordinarily termed critical damping. The manner of operation for operation with direct-current or rectified alternating-current track pulses is thus the same as that explained in detail in connection with Fig. 2A. Condenser 130 and Aresistors 131 and 132 shown in Fig. 4A perform the same function as that performed by condenser 29 and resistors 32 and 33, respectively-shownin Fig. 2A.V

As in theV two previousk forms of the invention shownY in Figs. 2A and 3A, -grid 'resistors 1133 and 134 are included in series with the control grids of 'tubes 135 and 1-3`6, respectively. These resistors are here also included to prevent undesirable oscillations of the tuned circuit in the eventl of intermittent short circuit between the plate and control 'grid' of either tub'esr135v and 136. A trap circuit is included between the plate and the B+ source of each of the tubes "andy 136. The trap circuit connected between the plate of the tube 135 and B+ includes condenser 137 and inductance 138. Similarly, the trap circuit connected between the plate of tube 136 and B+ includes' condenser 139 and inductance 140. These trap circuits perform the 'same functions as the corresponding trap circuits already mentioned in connection with Figs. 2A and 3A.

When the selector lever VSL is moved to the dotted line position, so that relay' SR becomes energized, front contacts 125, 126, and 127 are closed thereby connecting condenser 14S and full Wave rectifier 146 in series across the receivers 128 and 129. As in the embodiment shown in Fig'. 3, aftrap'circuit includin'g'condenser 147 and inductance 148 is connected 'across the input terminals of rectifier V146 aud is similarly used Lto aid in the rejection of interfering frequencies. By tuning this trap circuit 'to resonance at the frequency of the Yinterfering currents,

these currents are prevented from being applied to Vthe input ofthe rectifier because 'of the low impedance shunt path provi'dedfor them through 'thetrap circuit. The'rectifier outputfthuslincludesa direct-current component and 'also lsolite ripple voltage havin'g'its -rn'ain component at 13 a frequency twice that of the track current because of the full-wave rectification as is shown in waveform B of Fig. 4B. Condenser 130 connected across the output of the rectiiier acts now as a ilter condenser and thereby reduces the ripple amplitude of the rectiiier output.

In this embodiment of the invention the pulse-forming means includes resistors 155, 156, 161, and 162 and also condensers 157 and 159. The manner of operation of this circuit organization is in general similar to that of the corresponding circuit means included in Fig. 3A.

Tubes 135 and 136 are provided with class A bias by cathode resistor 149 which is by-passed by condenser 150. Since the cathodes of both tubes 135 and 136 are connected through resistor 149 to the junction of the resistors 131 and 132, the grid-cathode voltages of tubes 135 and 136 are of opposite polarity. Also, since resistors 133, 134, and 149 are of relatively low resistance, substantially all of the input voltages appearing across resistors 131 and 132 also appear between the grid and cathode of the corresponding tubes 135 and 136.

Because resistors 131 and 132 are connected in series across the rectifier output, the input voltages to tubes 135 and 136 each appear similar in form to that of the rectier output shown in Waveform B except that the grid-cathode voltages of these tubes are of opposite polarity. The similarly appearing output voltages appear across the respective load resistors 155 and 156. Assuming a particular set of connections, the grid-cathode voltage of tube 135 suddenly is driven more negative at the beginning of a code pulse so that the plate voltage of this tube ncreases. Because the voltage across condenser 157 cannot instantly be changed, this increase in plate voltage of tube 135 produces a corresponding increase in the grid-cathode voltage of tube 158. The grid-cathode voltage of tube 136 is at the same time increased so that the plate voltage of this tube decreases. The voltage across condenser 159 similarly cannot suddenly be changed so that a corresponding decrease in potential occurs between grid and cathode of tube 160. Following the sudden changes in potential at the plates of tubes 135 and 136, condensers 157 and 159 charge through resistors 161 and 162, respectively. As these condensers become charged, the charging rate decreases exponentially thereby reducing the magnitudes of the voltage variations produced on the control grids of'tubes 158 and 160. Consequently, during each on period of the track code the grid-cathode voltage of these output tubes 158 or 160 includes the ripple components continually present during each such on period but with its average value rising abruptly at the beginning of the code pulse and then decaying again towards zero as illustrated in waveform C. At the end of each track code pulse, a similar appearing voltage variation of opposite polarity is produced but without, of course, the ripple components because the track current is nonexistent at that time.

Because the control grids of tubes 158 and 160 are connected through equal resistors 161 and 162, respectively, and through battery 163 to ground, and because the cathodes of tubes 158 and 160 are both connected to ground the grid-cathode voltages of these tubes are not only of substantially equal magnitude, but also of opposite polarity. Thus, if the grid-cathode voltage of tube 158 is suddenly driven less negative, for example, the gridcathode voltage of tube 160 is simultaneously driven more negative by substantially the same amount.

Tubes 15,8 and 160 are both biased to cuto because of the connections of their control grids through resistors 161 and 162 to the negative terminal of a source of bias voltage 163. If, through a particular set of connections, the grid-cathode voltage of tube 158 becomes less negative at the beginning of a track code pulse so that the grid voltage of this tube might be represented by waveform C of Fig. 4B, then tube 158 conducts at the beginning of each track code pulse as its control grid is driven less v negative. The upper winding of relay CR'is thus ener- 1'4 gized and its armature actuated to a corresponding position. At the end of each track code pulse, the conduction' of this tube is not aiected since the driving of its control grid negatively below the quiescent cutoi point cannot change the conduction of this tube. The voltage applied to the control grid of tube 160, on the other hand, is of the opposite polarity to the voltage applied to the grid ofV tube 158. The conduction of tube is, therefore, not aected by the beginning of a track code pulse when its grid-cathode voltage is driven negatively beyond cutoff, but this tube does conduct at the end of each track code pulse when its gridis driven in the positive direction. The lower winding of relay CR is then energized and the relay armature actuated to the opposite position. The armature of this relay will thus be actuated to one position and then the other at a rate determined by the coding rate of the track current. Consequently, the decoding apparatus, such as that associated with the contacts of relay CR, is actuated and causes the proper cab signal to be displayed.

In this form of the invention shown in Fig. 4A, capacitors 157 and 159 perform two functions. When lever SL is moved to the D. C. position, the plate outputs of tubes 135 and 136 comprise the distinctive output voltages produced as a result of the shock excitation of la tuned circuit. Capacitors 157 and 159 should then preferably be relatively large to present a low value of impedance to the pulsing output of tubes 135 and 136 so that most of the voltage drop will occur across the respective grid leak resistors 161 and 162 and thus be as useful gridcathode driving voltage of tubes 158 and 160. However, when lever SL is moved to the A. C. position for operation with alternating track current, the plate output of tubes 135 and 136 comprises pulses having a duration substantially equal to that of an on period ofthe track code. To provide distinctive output voltages in response to these output pulses of tubes 135 and 136, condensers 157 and 159 must be relatively small so that they may become fully charged within a relatively short time following the beginning and end of each track code pulse. Despite these seemingly divergent requirements, in practice a satisfactory value for capacitors 157 and 159 has been found by experimentation to give the desired results for either alternating or unidirectional track current.

Decoding apparatus y Each time that code receiving relay CR included in' each embodiment of the invention is actuated to close its front contacts and 171 (see Pig. 4A), front repeater relay CRFP is energized lthrough a circuit including these contacts 170 and 171. lf relay CR is actuated to its alternate positions at a rate corresponding to that at which direct-current pulses are applied to the track rails, front repeater relay CRFP will remain steadily picked up despite its intermittent energizaton because of its slow releasing characteristics. Front contacts 172 and 173 of relay CRFP will then be closed. Therefore, each closure of back contacts 170 and 171 of relay CR will energize back repeater relay CREP. Since this relay is also provided with slow releasing characteristics, relay CRBP will also remain picked up despite its intermittent energization. This arrangement of front and back repeater relays of the code receiving relay CR ensures, by the picking up of relay CRBP, that relay CR is actually energized to opposite positions in response to the code pulses appearing on the track relays and is not steadily held in one position.

Contact 174 of relay CR, together with transformer 175, condenser 176, transformer 177, and rectifier 178 provide a decoding circuit that enables relay 180R to be picked up whenever relay CR is energized at the 180 rate. The form of decoding circuit shown in Fig. 4A is typical of that generally employed in coded track circuit signalling systems. Briey, eachtime that contact 174 of relay CR is energized to one position or the other,

one-half of the primary winding of transformer is 1'5 energiaedtthrough .-a circuit including this .contact 17,4 and' also `front Contact .179 of back :repeaterrelay CRB?.A The secondary windingiof transformer 175 is shunted by a series .resonant circuitl including condenser 76 and the primary vwinding of Atransformer 177. This series resonant circuit is .tuned to 180 cycles .per minute with the result that the secondary 'winding of transformer 177 is lenergizedonly when the primary winding of transformer 175 is energized `at the 180 rate. The .alternating `voltage vappearing across the lsecondary winding Vof transformer 177 is then rectified by full wave rectiler 178 so `that' relay 189K fis energized with -direct current. It

relay CR is .not intermittently .actuated to opposite posiv tions in the .180 ratein. response to a 180 code applied to the .track rails, relay -lSilR -will not be energized.

When-relay CR is energized in response to a 180 code,

both 4trout contacts 1180 of relay CRBP and v18,1 of relay 180B vare icloserl. Lamp 182 .of `*the cab -signal `is ythen energized to display a green aspect. If relay CR is en-Y ergized at the 75 rate, however, relay CRBP fis stilleny ergized but relay lSR lis dropped away. In that eventJV lamp 183 yof the cab signal is then energized through through Iback contact .i5-0 of relay CRBP and back con-V tact 1840i 'relay SQR to energize lamp 18S of thecab signal. A red aspect lis then displayed bythe cab signal.

By means of the circuit organization shown in Fig. 4A Y a cab signal may be selectively energized to display a Y proper signal Vaspect dependent upon the rate at which the CR relay is energized. yAlthough the 180 and 75 rates have been chosen las indicative of clear and-caution signal conditions, other. code rates may, offcourse, be used as desired. By'using-additional code rates, a larger number of diiercnt 'signal aspects may be displayed by the cab signal. Y

f The cab signalling -s-ystem thus provided by this invention permits vproper operation of the train-carried apparatus vwith various kinds of track circuit energization. In addition, the circuit organizaticnis so devised as to prevent an improper signal aspect from being displayed under various conditions of failure of the apparatus. For example, as already ekplained, the resistors connected inseries with the control grids of the two tubes in the yirstpstage of-amplication prevent oscillaticnsin the input tuned circuit in the event of an intermit- Vtent short circuit between the grid and plate of either of these tubes. Furthermore, the use of two channels of amplification with each channel cmtrolling .the ener-gization of .a .corresponding winding :of relay CB also contributes to 'the sair'ety .of this system. More specifically,

any Vintermitting noccurring Acircuittaaxlt Vaffecting bothtu es of the first .amplifier .stage simultaneously cannot cause :relay operation. Thus, it the common cathode connection 1ct the twotubes in the rstamplier Vstage is intermittently opened, the plate voltages of these tubes rises simultaneously. The .grid-cathode voltagesof both tubes :in .the second amplifier .stage would then both become less .negative simultaneously causing both tubes lto conduct V.plate 'current 'through .their respective Vrelay wind ings. when beth relayiwjndings :are simultaneouslyeniergi .the.code-responsivc relay'CR-"cannot'be picked Eire trap 'circuitsinctuded between the plate' of each of the input ampliertnbe's and Blthave been described as primarily preventing improper operation in the event that the apparatus `is used lin regions where rectified al.

ternating currents fare -use'd to energize the track circuits. Thesev trap circuits may, under certain circumstances, highly useful when the apparatus is used in re# et either direct-current or alternating-currenttrack cuergization. Thus, .it'the direct current for the track circuits isfobtained by rectiiicatio'n ofalternating current instead :ofilby :a :battery lascis shown in Fig. l, thetrap:

circuits willthen properly tuned, preventtherippk ofthe direct currentzpulses Vfrom'causing :chatter 'ofthe relay.

Also, -wl1'en operated in regions of either direct or alter-V nating-current track circuit energization, Vthese Vtrap :ciracuits may-aid in preventing stray currents in `the rails as. from lcommercial power lines from adversely affectingy the operation of the apparatus.

Having Adescribed three forms of code detecting-and;

decodingy apparatus ffor use withl a cab signalling system Yas specific embodiments of the present invention these forms have beenselected to* facilitate the disclosure ofV sections and additionalmeans for applying pulses of altermating `current `to the rails of other of said track sections,

vel'Jilcle-carried apparatus including receiver coils positioned-adjacent the rails of said track section, rst circuit means including a condenser to tune said receiver coils to .a relatively low frequency as `compared to the usual, power frequencies and damping meansfor damping the electrical oscillations in said receiver coils to ,provide distinctive output voltages of opposite polarity corresponding'respectively to .the beginning and end of each unidirec- ,tional track current pulse, vsecond circuit means including acondenser .to tunesaid receiver coils to vthe frequency of said alternating currentaand. also including vmeans for providing distinctive voutput voltages of opposite polarity corresponding respectively' tothe beginning and end of each valternating track current pulse, manually operable selector means for Vassociating either said rst or said .sec-

ond circuit means with Jsaid receiver coils, circuit means.

including electron discharge tubes for amplifying the distinctive output voltages' .provided by either said lirst or said second circuit means, relay'means including a twon position frelay operable to leither of its two diierentposi# tions and actuated alternately to its opposite positions in response to said distinctive voltages, 4and decoding means governed by said Vrelay'r'neans and selectively operated according to the rate of codiu'g'of either said unidirec-l tional or said alternating track current.

2. -A cab signalling system 'for railroads comprising, a plurality of track sections with means for applying pulses ot unidirectional current tothe rails of certain of vsaid trac'ksections and additional means for applyingpulses ot alternating/current to therrailsY of other of lsaid trackV sections, said current pulses being applied to said rails at selected rates according 'to trahie conditions, vehicle.- carriedapparatusincluding receiver coils ypositioned adjacent the rails of saidtrack sections, iirst pulseLformingcircuit means including rst tuning and `damping resistance meansresponsive to said'unidirectorraltrack current for producing"'distinctive k:pulses ofgoppositc Vpolarity corre spendingA respectively `'to thebe'ginning and end of each-,V track code-spinse; secondV Np'ul'seifel-ming Y'means includmg` second'tunitig'and"damping 'resistance meansl responsive'Y to said alternating tiackicnrrent also fr'or produ'chig "a ttnettveontpnt vonage 'stone A`polarfty at thebeginningpt each .track Icode A pulse and another distinctive 'output voltage' of opposite polarity at the 'end of'each track code pulse, circuit means including electron discharge tubes, selector means for connecting either said rst or said 'second pulse-forming means between` said receiver coils 'andA said circuit means including -sai'd elect-ron tubes,l relay opposite positions in respons'eto said 4distinctive voltages,

l? and decoding means.' governed by Said relay means ,and selectively operated-according to the coding rate of the track current. g

3. A cab signalling system for railroads comprising, a plurality of track sections, circuit means for applying pulses of unidirectional current to the rails of certain of said track sections and additional circuit means for applying pulses of alternating current to the rails of others of Said track sections with said pulses applied at selected rates according to trac conditions, Afirst pulse-forming circuit means including first tuning and damping resistance means responsive to said unidirectional track current and second pulse-forming circuit means including second tuning and damping resistance means responsive to said alternating track current to provide distinctive voltage variations of opposite polarity corresponding respectively to the beginning and end of each code pulse, circuit means including electron tubes for amplifying vsaid dist'mctive voltage variations, manually controlled means for selecting either said first or said second pulse-forming means to cooperate with said receiver coils and saldamplifying means, relay means including a two-position relay operable to either of its two different positions and actuated by said amplifying meansalternately to its opposite positions in response to said distinctive voltages, and decoding means selectively controlled by the rate of 'actuation of said relay means.

4. A cab signalling system for railroads comprising, a plurality of track sections with means for applying pulses of alternating current to the rails of said track sections at any one of various selected rates according to trac conditions, train-carried apparatus including receiver coils mounted adjacent the track rails, tuning means connected with said receiver coils to resonate said coils to the frequency of said alternating track current, rectifier means associated with said` receiver coils to rectify the alternating current in said coils, pulse-forming means including tuning and damping resistance means responsive to the unidirectional voltage provided by said rectifier means to produce distinctive voltage outputs of opposite polarity corresponding respectively to the beginning and end of each track code pulse, amplifier means including electron discharge tubes for amplifying said distinctive output voltages, relay means including a two-position relay governed by said amplifying means and being operated alternately between its two positions, and decoding means governed by said relay means and selectively operated according to the coding rate of said alternating track current.

5. ln a cab signalling system 'for railroads, a plurality of track sections and means for applying pulses of alternating current to the rails of said track sections at dierent selected rates according to traffic conditions, vehiclecarried apparatus including receiver coils positioned adjacent the rails of said track sections, a lirst condenser and a rectiiier connected in series with said coils to respectively tune said coils to the frequency of said alternating track current and to rectify the current through said coils, a transformer having its primary winding energized with the current rectified by said rectifier, a second condenser connected across the secondary winding of said transformer to thereby provide an oscillatory circuit having a natural frequency relatively low as compared to the usual commercial power frequencies, circuit means for damping the electrical oscillations of said tuned circuit as it responds to the unidirectional output voltage of said rectifier to provide distinctive output voltages of opposite polarity corresponding respectively to the beginning and end o cach track code pulse, circuit means including electron tubes connected with said secondary winding for amplifying said distinctive output voltages, relayV means including a two-position relay governed by said amplifier means and being operated alternately between said two positions, and decoding means selectively operated by said relay means according to the coding rate of the track current.

6. A cab signalling system for railroads including, a

plurality of track; sections with rneansfor applying pulses of `unidirectional, current to the rails of certain of said track sections and additional means for applying pulses of alternating current to the rails of other of said track sections with said current pulses being applied to said rails at selected rates according to traflc conditions, vehicle-carried equipment comprising receiver coils positioned in inductive relationship to said track rails, a rst condenser to tune said receiver coils to a relatively low frequency as. compared to the usual commercial power frequencies, damping means associated with said coils for damping the oscillations appearing across said coils so'that a distinctive output voltage of one polarity is provided at; the beginning of each code pulse and another distinctiveyoltage of opposite polarity at the end of each codepulse, a second condenser to tune said coils to the frequency-of said alternating track current, a rectier to rectify the alternating current in said coils,- pulseforming means including tuning and damping resistance means associated with said second condenser and rectifier to provide a distinctive output voltage of one polarity at the beginning of each track code pulse and another distinctive output voltage of opposite polarity at the end of each code pulse, manually operable means to connect said lirst condenser and said damping means with said coils when said vehicle is operated over track circuits energized with unidirectional track current and to connect said second condenser and' associated rectifier and pulse-forming means with said coils when said vehicle is, operated in regions of alternating current track circuit energization, amplifier means including electron tubes to amplify said distinctive output voltages, relay means including a two-position relay governed by the output of said amplier means and being actuated alternately between its opposite positions at a rate corresponding to the coding rate of said track current, and decoding means selectively operated according to the coding rate of said track current.

7. In a cab signalling system for railroads, a plurality of track sections and' means for applying pulses of altermating currentv to the rails of saidl track vsections at selected rates according to traic conditions, train-carried equipment comprising receiver coils mounted adjacent the track fails, a first condenser connected with said receivers to tune said receivers to the alternating track current frequency, a rectifier connected with said coils to rectify the alternating Current through said coils t0 provide a unidirectional output voltage across said rectifier, a parallel-connected inductance and second capacitance tuned to a relatively low frequency as compared to the. usual commercial power frequencies, circuit means connecting 'said rectifier output with said induetance to shock excite the tuned circuit formed by said inductance and said second capacitance by said unidirectional output voltages provided by said rectifier, means for damping the oscillations of said tuned circuit so that a distinctive output voltage of one polarity is provided corresponding to the beginning of each track pulse and another distinctive output voltage of opposite polarity is provided corresponding to the end of each track code pulse, amplifier means associated with said tuned circuit for amplifying said distinctive output voltages, relay means including a two-position relay controlled by said amplier means and being operated alternately between its opposite positions, and decoding means selectively controlled according to the rate of track current coding.

8. A cab signalling system for railroads comprising, a plurality of track sections and means for applying pulses of alternating current to the rails of said track sections at different selected rates according to traic conditions, train-carried apparatus including receiver coils mounted in inductive relationship to the track rails, a rst condenser and a rectier connected in series with said coils to respectively tune said coils to the frequency of said alternating track current and to rectify the current through said coils, a parallel connected inductance and second capacitance, coupling circuit means for applying the unidirectional output voltage provided by said rectier across said inductance to shock excite the tuned circuit formed by said inductance and second capacitance, circuit means for dampening the electrical oscillations of said tuned circuit thereby providing a distinctive output voltage of one polarity corresponding to the beginning of each track code pulse and anotherV distinctive voltage of opposite polarity corresponding to the end of each track code pulse, trap circuit means including a series resonant circuit associated with said tuned circuit for removing undesired ripple voltages from the output voltages provided by said tuned circuit, circuit means including electron tubes connected with said tuned circuit for amplifying said distinctive output voltages, relay means including a two-position relay governed by said amplifying means and being operated alternately between its opposite positions, and decoding means selectively operated by said relay means according to the coding rate of the track current.

9. In a cab signalling system for railroads, a plurality of track sections with means for applying pulses of unidirectional current to the rails of certain of said track sections and additional means for applying pulses of Valternating current to the rails of other of said track sections with said current pulses applied to said rails at selected rates according to traic conditions, vebicle carried apparatus including receiver coils positioned adjacent the rails of said track sections, rst pulse-forming circuit means including first tuning and damping resistance means responsive to the unidirectional current for producing distinctive pulses of opposite polarity corresponding respectively to the beginning and end of each track code pulse, second pulse-forming means including second tuning and damping resistance means vresponsive to said alternating track current for producing distinctive output voltages of opposite polarity corresponding respectively to the beginning and end of each track code pulse,

amplifier means comprising two channels of amplification with each of said channels including an electron tube biased to cutoff, manually operable selector means for connecting either said first or said second pulse-forming means between said receiver coils and said amplifier means, coupling circuit means for applying with opposite polarity respectively said distinctive output voltages between grid and cathode of said electron tube, relay means including a two-position relay operable to either of its two different positions and having two windings, a platecathode circuit of each of said electron tubes including a winding of said relay means whereby said relay means is actuated alternatingly between its opposite positions 20 in vresponse to said distinctive voltages, and decoding means governed by said relay means and selectively operated according to the rate of coding of said track current.

10. A cab signalling system for railroads comprising, a plurality of tracksections with means for applying pulses of alternating current to the rails of said track sections at difr'erent selected rates according to trafiic conditions, vehicle-carried apparatus including receiver coils positioned adjacent the rails of said track sections, a tuning condenser andrectiiier connected in series with said receiver coils to respectively tune said coils to the frequency of said alternating track current and to rectify the alternating current in said receiver coils, pulse-form ing circuit means including, a discharge resistor connected across the output of said rectifier, two charging capacitors each-having a terminal connected to a terminaltof said discharge resistor, two equal series-connected resistors each having a terminal connected respectively to the remaining terminals of said charging capacitors, amplifier means including two electron tubes each having plate and cathode and control grid electrodes, coupling circuit means for applying the voltage across each of said equal resistors between the control grid and cathode of a cort responding electron tube, whereby at the beginning of each track code'pulse said rectier charges said charging capacitors to one polarity through said equal resistors to produce a momentary voltage output ofV one polarity across said equal resistors and at the end of each track code pulse the drop of rectifier output voltage discharges -said charging capacitors through said equal resistors and said discharge resistor to produce a momentary output voltage across said equal resistors of opposite polarity,

code-following relay means including a two-position relay Y controlled by said ampliiier means and being operated alternately between its opposite positions, and decoding means selectively controlled by said relay means according to the rate of coding of said alternating track current.

References Cited in the file of this patent UNITED STATES PATENTS ,...r man, 

